1. Field of Invention
The invention relates to a method for forming a portion of a workpiece, such as a cylinder or shell, and an apparatus therefor, especially the method and apparatus for forming the portion of the workpiece by spinning to form a changed diameter portion having a central axis that is at least one of offset from and oblique to a central axis of an unprocessed portion of the workpiece.
2. Description of Related Art
In Japanese Patent Laid-open Publication No. 3-226327, disclosed is a method for forming an end portion of a cylindrical member (hereinafter, simply referred to as a cylinder) made of metal to form a reduced diameter portion on the end portion. According to the Publication, a spinning process is performed by supporting the cylinder with a chuck and rotating it about its axis, and moving a roller for forming toward the axis to reduce the diameter of the cylinder, thereby to form the reduced diameter portion having a neck portion and a tapered portion. In general, the spinning process is employed to form a plate into a shell. Likewise, a flange and neck portion can be formed by spin flow forming into a cylindrical can body, as disclosed in U.S. Pat. No. 4,563,887. Furthermore, a computerized spinning machine has been proposed in Japanese Patent No. 2,534,530.
Recently, it has been requested to form a reduced diameter end portion having an oblique axis inclined against the central axis of the metal cylinder. When the metal cylinder is used for an outer shell of a muffler of an automotive vehicle, for example, the cylinder will be easily mounted in a vehicle. Also, when the metal cylinder is used for a housing of a catalytic converter, it will be easily located near an engine, to reduce the time of increasing the temperature of the catalyst. Furthermore, dual converters may be easily assembled, with their neck portions positioned close to each other.
According to the prior methods for forming the cylinder or shell by the spinning process, the reduced diameter portion was formed to be coaxial with the main body of the cylinder, but the reduced diameter end portion having the oblique axis could not be formed. In order to produce the cylinder like the shell or housing as described above, therefore, the portions corresponding to the main body and the reduced diameter portion were formed by press working, and then these components were connected together by welding or the like. According to these methods, however, the produced cylinder can not be expected to be so strong, comparing with that of the integral construction. Furthermore, they need the connecting process, different from the forming process, so that it is difficult to produce the cylinder by those methods, and it is almost impossible to produce the cylinder by the computerized forming process as described in the prior publication. As a result, the manufacturing cost of the cylinder shall be increased, comparing with the cylinder of the coaxial type formed by the spinning process.